This invention relates to a screw grommet molded of a plastic material and intended for fastening as onto a metal plate another metal plate, a hook or other similar article by means of a screw. As a screw grommet of the type designed for fastening a given article with a screw onto a metal plate, it has heretofore been known to use a plastic screw grommet which comprises a flange head, a leg having a polygonal (generally tetragonal) section and perpendicularly extending downwardly from the underside of the flange head, engaging projections formed on the sides of the leg and tapering toward the extremity of the leg, a screw aperture dug inwardly from the topside of the head along the axis of the leg and grooves cut radially into the inner wall of the screw aperture.
As is universally known, this screw grommet is fastened to a given metal plate by having the leg thereof inserted into an aperture somewhat larger than the cross section of the said leg and perforated in advance through the metal plate, causing the engaging projections formed on the sides of the leg to be forcibly slid in past the edge of the said aperture by virtue of the elasticity of the material of which the grommet is made until the engaging projections are brought into snug engagement with the rear surface along the edge of this aperture and consequently immobilized against the metal plate and thereafter driving a prescribed setscrew into the screw aperture.
By virtue of this engagement of the engaging projections, therefore, the screw grommet itself gets fastened fast onto the metal plate. In order that the setscrew may be fastened with desired tightness to the screw grommet, however, this screw grommet is generally made of a rigid plastic material. If the said engaging projections are increased in their degree of protuberence, therefore, they offer unyielding resistance to the edge of the aperture in the metal plate in the process of the grommet's insertion into the aperture, defying all efforts to accomplish the insertion. For this reason, the engaging projections are inevitably formed in limited dimensions and therefore fail to provide sufficiently fast engagement. Thus, it is always found necessary that the diameter of the aperture for insertion in the metal plate and the diameter of the leg of the screw grommet are adjusted to each other to ensure safe insertion and fast engagement.
It is an object of this invention to provide a plastic screw grommet capable of providing easy engagement with the aperture perforated in the metal plate and enabling two metal plates to be fastened fast to each other.